Loss of Calpain 3 dysregulates store‐operated calcium entry and its exercise response in mice

Limb‐Girdle Muscular Dystrophy R1/2A (LGMD R1/2A) is caused by mutations in the CAPN3 gene encoding Calpain 3, a skeletal‐muscle specific, Ca 2+ ‐dependent protease. Localization of Calpain 3 within the triad suggests it contributes to Ca 2+ homeostasis. Through live‐cell Ca 2+ measurements, muscle mechanics, immunofluorescence, and electron microscopy (EM) in Capn3 deficient (C3KO) and wild‐type (WT) mice, we determined whether loss of Calpain 3 altered Store‐Operated Calcium Entry (SOCE) activity. Direct Ca 2+ influx measurements revealed loss of Capn3 elicits elevated resting SOCE and increased resting cytosolic Ca 2+ , supported by high incidence of calcium entry units (CEUs) observed by EM. C3KO and WT mice were subjected to a single bout of treadmill running to elicit SOCE. Within 1HR post‐treadmill running, C3KO mice exhibited diminished force production in extensor digitorum longus muscles and a greater decay of Ca 2+ transients in flexor digitorum brevis muscle fibers during repetitive stimulation. Striking evidence for impaired exercise‐induced SOCE activation in C3KO mice included poor colocalization of key SOCE proteins, stromal‐interacting molecule 1 (STIM1) and ORAI1, combined with disappearance of CEUs in C3KO muscles. These results demonstrate that Calpain 3 is a key regulator of SOCE in skeletal muscle and identify SOCE dysregulation as a contributing factor to LGMD R1/2A pathology.